The illnesses began about 11 p.m. Wednesday in a female dormitory housing about 90 women, the York Daily Record newspaper reported on its website. By early Thursday, 49 inmates had been taken to hospitals. All the inmates had been returned to the prison by Thursday afternoon, the county said in a statement, according to the newspaper.

The heating system was shut down, and the county's statement said carbon monoxide levels had returned to normal.

Prison Warden Mary Sabol said prison officials would meet with the McClure Co., the company that works on heating at the prison, and would consider installation of carbon monoxide detectors, the Daily Record reported.

About 215 women are incarcerated at the facility, 85 miles west of Philadelphia.

Prisoners living in the affected unit have been relocated to other areas in the facility.